The combustion chamber pressure is an important parameter which provides information on the progression of combustion in the combustion chamber of an internal combustion chamber. Variations of the instant of ignition, the mixture composition, and the mixture distribution have an effect on the progression of the combustion process in the engine and on the pressure which is generated therein. In particular, when control of the above parameters is unfavorable, engine knocking can occur, which is very disadvantageous for the operation and the life of the engine. There are also related effects on the composition of the exhaust gases and the efficiency of the engine.
The detection of knocking is thus of great interest in regulating the mixture composition and/or the instant of ignition. Detecting knocking with the aid of torque meters, acceleration meters or microphones is already known. However, such methods require frequency filters which permit the passage of a frequency which is specific for knocking. Knocking can not be extracted as a pure signal in this manner, but only as a derived signal.
The detection of knocking with the aid of an ionic current sensor is also known. It has thereby been ascertained that the output signal of the ionic current sensor has low and high frequency components and that the high frequency components increase virtually in uniformity with the intensity of engine knocking. The difficulty has been the separation of the low frequency ionic currents from the high frequency ionic currents which are specific for knocking. To this end, an RC high-pass filter was provided in the known apparatus, with a subsequent ammeter which indicates the average strength of the current of ionic current components which have been thus filtered out. However, this apparatus is very much dependent upon the mean effective pressure prevailing in the combustion chamber.